The present invention relates to nuclear fuel assemblies and more particularly to lock down devices for nuclear fuel assemblies.
It is well known that nuclear power reactors typically contain a plurality of nuclear fuel assemblies arranged and supported between an upper core alignment plate and a lower core support plate. The upper and lower ends of the fuel assemblies are typically provided with end fittings which include alignment posts that extend outwardly from the end of the fuel assemblies and slidably engage post receiving openings in the plates. The weight of the fuel assemblies is typically borne by the core support plate.
During operation of the reactor a fluid coolant such as water is forced vertically upward through the fuel assemblies to remove the heat generated therein. This upward flow produces a lifting force on the fuel assemblies which can exceed the weight of the fuel assembly itself. Consequently, various prior art devices have been used at the top or the bottom of the fuel assembly to prevent the assembly from lifting off the core support plate. If this upward motion of the fuel assembly is not prevented, damage to its fuel rods and to the upper alignment plate may result. In addition, lateral motion of the fuel assemblies can adversely affect the local power density in the core and can cause wear between the fuel rods and the grid structure of the fuel assembly.
The devices which hold the fuel assemblies in place must also accommodate differential thermal expansion of each assembly and the various other core components. Thus the fuel assemblies are usually supported between the upper alignment plate and the core support plate in a manner which permits relative axial growth without overly stressing the components of the fuel assembly.
Although, as described above, the fuel assembly must be held firmly in place during core operation, the periodic refueling of the reactor requires that the assembly be easily relocatable within the core when the reactor vessel head is unbolted and removed.
One method of holding down fuel assemblies disclosed in the prior art uses a bayonet lock at the bottom of the center fuel element of a seven element hexogonal assembly. The lock is actuated by rotating the center fuel element. This device is impractical in modern reactors where closely packed square fuel assemblies do not provide clearance for rotation. Other bottom mounted locking devices permit simultaneous locking or unlocking of an entire row of fuel assemblies through the motion of an actuating rod that extends horizontally and in contact with a bottom extension of each fuel assembly. Each actuating rod is driven by means external to the reactor vessel thus requiring many penetrations in the lower portion of the reactor vessel. These penetrations are undesirable for reactors operating at the high pressures common in modern reactors, and external driving means require space around the reactor vessel which is not provided in current reactor cavity designs.
In recent years the major nuclear power reactor suppliers in the United States have held assemblies in place with the use of various spring arrangements between the top of the fuel assembly and the upper core alignment plate. These designs typically require a substantial compressive pre-loading of the springs so that enough hold-down force is applied to the assemblies to resist the upward forces that exist during core operation. This pre-load force is transmitted from the top of the fuel assembly to the bottom of the assembly through the control element guide tubes. The guide tubes provide a path for insertion of the control rods, and the guide tubes and the fuel assembly spacer grids attached thereto provide the framework which maintains the proper spacing and alignment of the individual fuel rods in an assembly.
In the past the control rod guide tubes were typically made from stainless steel, a material having the desirable characteristic of high compressive strength, but the undesirable characteristic of a high cross section for parisitic neutron absorption. In order to improve the neutron economy in the reactors, the guide tubes have more recently been fabricated from zircaloy. This material is less resistant to compressive stresses than is stainless steel. Consequently, there is a greater likelihood that the pre-loading of the fuel assemblies as described above can result in bowing of the guide tubes and as a result bowing of the entire fuel assembly. This problem has become more acute in recent years as the power densities and flow rates in the reactors have increased, thereby requiring larger hold-down springs and pre-loading forces. These top mounted springs have several other disadvantages. Their size not only contributes to the flow resistance offered by the fuel assembly, but also increases the required height of the reactor vessel by several inches and the cost of the vessel by several thousand dollars. In addition, since the pre-load force typically originates from the weight of the reactor vessel head and upper guide structure bearing down on the upper core alignment plate, if the total pre-load which the upper springs must provide is greater than the combined weight of the reactor vessel head and upper guide structure, special techniques are required for bolting down the reactor vessel head.
A prior art improvement to the top mounted spring hold-down device moves the springs to the bottom of the fuel assembly where an upward compressive force is applied to the bottom of the control rod guide tubes. The compressive pre-load force on the guide tubes in this design is approximately equal to the weight of the fuel assembly in water, or about 1200 pounds. This is a significant reduction in compressive force relative to the upper spring design but the possibility of guide tube and fuel assembly bowing is still significant. The consequences of fuel assembly bowing can be quite severe. For example, the local power density surrounding individual fuel pins can be much higher than predicted under unbowed conditions. Also a bowed fuel assembly may not be relocatable because the distortions and non-rectangularity may prevent a proper fit in relation to adjacent assemblies. A further possibility is that the guide tube will be bowed enough to interfere with the dropping of a control rod therethrough.
Another disadvantage of both the top mounted spring hold-down and the bottom mounted spring hold up devices disclosed in the prior art is the possibility of small but continual lateral motion of the end fittings and guide tubes during core operation. Since the fuel assembly spacer grids are attached to the guide tubes, these grids also will move laterally. The lateral motion of the grids can cause wear on the surface of the fuel rods which, after a year or two of operation, may significantly increase the susceptibility of the fuel rods to clad failure.
Thus it can be seen that early fuel assembly hold-down devices included positive locking means at the bottom of the fuel assembly. However, as reactors became larger and more complex, space limitations and ease of refueling, in combination with the ability to accommodate different expansion rates of the various core components, led to the wide spread use of upper mounted spring hold-down devices and, more recently, lower mounted spring hold up devices. Although these latter devices adequately perform their intended hold-down function, they increase the possibility of problems resulting from compression of the control rod guide tubes and the small but continual lateral motion of the guide tubes.